I've got an ongoing algae battle and I think I'm just not slamming the pool hard enough..

The basics..12 by 30 inground with a vinyl liner and sand filter. I use the slow dissolve tabs. Figure it holds around 14,000 gallons.

The PH has been around 7.2, alkilinity was 80.

CYA was 50. I've never known my Cl to be below 4 or 5.

I had an algae bloom start and added 2 pds of powdered Cl. Algae seemed to go away, cleaned the pool, had water tested and was told it was all perfect. Algae returned...2 more pds of Cl shock..algae gone but CL was off the charts. Water was clear - main drain was perfectly visible - but water had dingy look to it. I figured I was dealign with a combined cl issue so this time I hit it with a pound and a half of monopersulfate. Overnight the water took on a sparkle. CL remained very high - like off the charts on my test kit - but water felt ok.

Less than 24 hours later I'm back to the algae precipitating out onto the bottom of the pool...and my CL is still off the charts.. Did I not shock the thing hard enough? It seems to me that I'm dealing with a combined chlorine problem and the organics from the algae are binding up whatever I put in the pool. How much shock - and what type - should I be adding to try and get this thing back to right?

The water has not been green - what I get is a flat look to the water and greenish "dust" on the bottom.

I don;t have current PH and alk numbers - local pool store says they can't retest until CL is down lower...and they only test for total chlorine not just free.